This invention is directed generally to a method for dissolving high molecular weight, water soluble polymers.
One embodiment of this invention is directed to the dissolving of water soluble polymers to form a liquid solution.
Another embodiment of this invention is directed to the dissolving of high concentrations of polymers in water and other solvents to form solid, homogeneous, essentially solvent-plasticized, composites and to the uses of those composites.
Although many of the high molecular weight water soluble polymers, the poly (ethylene oxide) polymers for example, are extremely soluble in water, nevertheless they are quite difficult to dissolve. The polymer particles adhere strongly to one another upon wetting forming clumps of undissolved polymer and partially dissolved gel lumps which are difficult to disperse and which require long agitation time to obtain a uniform solution.
While the dissolving rate can be increased either by use of high temperatures or intense agitation, these practices are undesirable. If high shear agitation is employed to enhance dissolving rate, the shearing forces cause rupture or breakage of the polymer chain thus reducing the molecular weight of the polymers in solution. Consequently, a number of specialized procedures and apparatus have been developed to prepare solutions of these polymers on a small batch scale and in a larger volume continuous manner. Bulletin No. F-42933 of the Union Carbide Corporation entitled "How to Dissolve Polyox Water-Soluble Resins" is illustrative of approaches taken in the prior art to the problems encountered in the dissolving of Polyox (a registered trademark of the Union Carbide Corporation for poly (ethylene oxide) resins). As noted therein, individual polymer particles must be dispersed throughout the liquid to avoid agglomeration and lump formation. For batch processes, a predispersion technique in which the polymer particles are first dispersed in a water-miscible non-solvent, gives good results. The non-solvent may comprise an organic liquid such as isopropanol, a relatively concentrated aqueous salt solution, or even boiling water as the polymer precipitates from water at or near the boiling point. After dispersing the polymer in the nonsolvent, water is added with continuous agitation to dissolve the polymer.
The main disadvantages of the predispersion technique are the introduction of foreign materials (non-solvents) into the final solution, the relatively cumbersome and time consuming procedures involved and the inability to obtain relatively concentrated polymer solutions. Solutions of high molecular weight polymers, e.g., those having a molecular weight above about 3,000,000 are extremely viscous semi-solids to solids at polymer concentrations above about 5%. The dissolving machines so far developed likewise are incapable of producing concentrated polymer solutions.
One important use of extremely dilute polymer solutions is for the reduction of hydrodynamic drag of water flowing through a conduit or of an object moving through water. This effect is well known and is utilized for such purposes as enhancing water flow and stream reach in fire fighting. Friction reductions as great as 70% or more have been reported for high molecular weight poly (ethylene oxide) concentrations of 50 ppm or less flowing through a pipe. However, as shown by a number of research papers, the extent of drag reduction obtained is a direct function of molecular weight, increasing as molecular weight increases, and is very sensitive to polymer degradation through shear effects. One exemplary paper is that of Ting et al, entitled "Characterization of Drag Reduction and Degradation Effects in the Turbulent Pipe Flow of Dilute Polymer Solutions" published in J. Applied Polymer Science, Vol. 17, pp. 3345-3356, (1973). As shown in that paper, repeated passes of the same sample solution through a pipe system resulted in continuing decreases in the observed drag reduction.
Rotary pumps, particularly high speed centrifugal pumps, produce much more severe shear forces than those incurred in the turbulent flow of liquids through a pipe. Consequently, molecular weight degradation of dilute polymer solutions is especially extreme in those systems pumping water at relatively high pressures and velocities such as those commonly employed in fire fighting. Yet the prior art has been unable to develop a practical technique for the continuous production of low concentration polymer-water streams without degrading the polymer by passing the solution through pumps.